Filter Circuit for an Electret Microphone

ABSTRACT

A miniature electret microphone includes an input buffer circuit. The input buffer circuit includes an integrated circuit including a floating ground substrate and a p-n junction. The p-n junction is coupled as a capacitor to a filter circuit of the input buffer circuit

TECHNICAL FIELD

This patent relates to the field of listening devices and in particularrelates to a filter circuit that may be used with an electret microphoneof a listening device.

BACKGROUND

Miniature electret microphones for use in hearing instrument (HI)devices often incorporate electronic filters in order to achieveimproved performance of the HI system. A low pass filter (LPF) can beused to address the susceptibility of the HI to high amplitude, lowfrequency disturbances such as wind and road noise that tend to saturatethe input buffer. A high pass filter (HPF) can be used to reject higherfrequency disturbances such as ultrasonic interference and switchingnoise from the signal processing component.

Alternatively, microphones for HI devices may incorporate mechanicalfilters, for example, screens or other structures within the audio portsof the device that provide a filtering affect. The physical screen,usually located in the input port of a microphone of the HI device maybe used to perform acoustic damping of the microphone's inherentresonance.

Use of electronic filters in place of mechanical, screen filters offersome performance advantages by reducing noise inherent with themechanical filter. Additionally, the electronic filter also allowsintegration of the filter components within other electronics of thehearing instrument.

Integrating filter components on an integrated circuit of the HI insteadof using discrete components on the circuit assembly is highly desirablefor manufacturability, reliability, and cost. There is a tradeoff in thevolume reduction of these components (predominately for the externalchip capacitors) and the area needed to integrate these components. Forvery small footprint microphones, it is important to minimize the volumeof the internal circuitry of the microphone as well as to reduce thestray capacitances loading the input. Reducing stray capacitance loadingof the input is also necessary to minimize the sensitivity loss of themicrophone.

To reduce the sensitivity loss as a result of stray capacitance in themicrophone, a floating substrate can be driven in such a way as to guardout the stray capacitance that exists from the charged back plate (inputof the buffer circuit) and the substrate of the die. Such an arrangementis shown in commonly owned U.S. Pat. No. 5,466,413, the disclosure ofwhich is hereby expressly incorporated herein by reference. Normally,the substrate of the die is grounded, and the stray capacitance presentsa capacitive load on the input to ground reducing circuit gain andoverall microphone sensitivity. This parasitic capacitance cansignificantly degrade the sensitivity of the microphone for very smallfootprint microphones because the die is very close to the backplate ofthe microphone and the motor capacitance driving the buffer input issmall (due to the small microphone package and manufacturingtolerances). The motor capacitance is on the order of a pico farad, sostray capacitances on the order of a hundred femto farads can reduce thesensitivity by a decibel (dB).

Floating the substrate and driving it with a buffered copy of the inputsignal can be used to guard out stray capacitance to reduce signal loss.The drawback of this method is increased circuit noise due to thefeedback of the guarded signal to the input. This further highlights theneed to minimize stray capacitance even when it is being guarded out.One way of minimizing this stray capacitance is by increasing the gapbetween the back plate of the microphone and the circuit assembly. Thisis not easily accomplished in a miniature microphone. Another way is byreducing the area of the integrated circuit die.

Integrated filter capacitors can be implemented using either thin filmcapacitors or by utilizing the depletion region of a p-n junction.Maximizing the capacitance per unit area assists the designer inminimizing the area of the die. Junction capacitors for integratedcircuits have been utilized in the prior art in grounded substratedesigns. However, because there is little or no benefit of using afloating substrate architecture in normal applications or largefootprint miniature electret microphones, they have not been used insuch applications.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the disclosure, reference should bemade to the following detailed description and accompanying drawings.

FIG. 1 is a graphic illustration of a microphone transducer adapted inaccordance with the preferred embodiments of the invention.

FIG. 2 is a circuit diagram illustrating incorporation of one or morefilters into the microphone circuit as shown in FIG. 1.

FIG. 3 is a schematic illustration of a silicon die incorporating one ormore filters structures in accordance with the preferred embodiments ofthe invention.

FIG. 4 is a schematic illustration of a silicon die incorporating one ormore filters structures in accordance with the preferred embodiments ofthe invention.

FIG. 5 is a schematic illustration of a silicon die incorporating one ormore filters structures in accordance with the preferred embodiments ofthe invention.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity. It will further be appreciatedthat certain actions and/or steps may be described or depicted in aparticular order of occurrence while those skilled in the art willunderstand that such specificity with respect to sequence is notactually required. It will also be understood that the terms andexpressions used herein have the ordinary meaning as is accorded to suchterms and expressions with respect to their corresponding respectiveareas of inquiry and study except where specific meanings have otherwisebeen set forth herein.

DETAILED DESCRIPTION

While the present invention is susceptible to various modifications andalternative forms, certain embodiments are shown by way of example inthe drawings and these embodiments will be described in detail herein.It should be understood, however, that this disclosure is not intendedto limit the invention to the particular forms described, but to thecontrary, the invention is intended to cover all modifications,alternatives, and equivalents falling within the spirit and scope of theinvention defined by the appended claims.

A miniature electret microphone that may be used in any number ofdevices for converting acoustic signals to electrical representations ofthose signals may incorporate electronic filtering as part of anassociated buffer circuit. The miniature electret microphone may beincorporated into, for example, a hearing instrument, such as a hearingaid, and the filtering/buffer circuit may be incorporated within the dieof an integrated circuit associated with the hearing instrument. Thereis, of course, no limitation on the application of the miniatureelectret microphone, which may find use in communication devices,computing devices or virtually any application that requires conversionof acoustic signals to electrical signals.

The buffer circuit for the miniature electret microphone may incorporateone stage or multiple stages of filtering, and for example, the buffercircuit may incorporate a low pass filter (LPF) and a high pass filter(HPF) defining a band pass. The LPF may be configured to reject highamplitude, low frequency disturbances such as wind and road noise, whichtend to saturate the buffer circuit. The HPF may be configured to rejecthigher frequency disturbances such as ultrasonic interference andswitching noise from the signal processing components. Advantageously,the HPF may replace the traditionally used physical/acoustic screenfilter disposed in the inlet port of the microphone. Such use of the HPFmay provide further benefit of reducing or eliminating input screenresistance noise providing an overall reduction in system noise withreduced manufacturing complexity.

FIG. 1 illustrates a miniature electret microphone 100 including amicrophone housing 102 and an inlet port 104 coupling an exterior of thehousing 102 to an interior 106. The interior 106 is divided by a backplate 108 and diaphragm 110 assembly into a front volume 112 and a backvolume 114. In this manner, the microphone 100 is of conventionaldesign, although it will be appreciated that the invention hasapplication to alternative constructions. As is well known, the backplate 108 and diaphragm 110 assembly form a capacitor the value of whichchanges responsive to changes in the relative position of the diaphragm110 to the back plate 108 responsive to sound pressure on the diaphragm110.

The miniature electret microphone 100 further includes a circuitassembly 116, for example a printed circuit board or suitable structureonto which one or more electric circuit components may be disposed andoperatively coupled. One such component, an integrated circuit 118 isshown operatively coupled to the circuit assembly 116. The circuitassembly 116 is suitably coupled to the back plate 108 by a wire bond120, for example, although other structure for coupling the back plate108 to the circuit assembly 116 may be used. Because of the chargednature of the back plate 108, stray or parasitic capacitance 122 may beestablished between the back plate 108 and the circuit assembly 116.

The integrated circuit 118 may incorporate, among other signalprocessing circuits associated with the microphone 100, a buffer circuit200. In the embodiment illustrated by FIG. 2, the buffer circuit 200includes a first buffer stage 202, a second buffer stage 204, a filter206 that may include a low pass filter (LPF) 208 and a high pass filter(HPF) 210 coupled between an input 212 and an output 214. The buffercircuit 200 further couples to a power source 216 and a ground orreference 219. The first buffer stage 202 may include first and secondinput transistors 220 and 222, first and second input diodes 224 and 226and resistor 250 and have an output MNOUT1. The first input transistor220 may be a depletion mode NMOS transistor that couples to the backplate 108 of the microphone 100 through terminal VIN. The input 212 isfurther biased with anti-parallel diodes 224 and 226 to ground 219. Thesecond buffer stage 204 may include first and second output transistors230 and 232 and resistor 234 and have an output MNOUT2. The first bufferstage 202 and the second buffer stage 204 may be separated by the filter206, which may be a band pass filter.

As shown in FIG. 2, the filter 206 includes the LPF 208 and the HPF 210forming the pass band. The LPF 208 may be located at the output ofeither one of the first buffer stage 202 or the second buffer stage 204.There are advantages and hence it is preferred to locate the LPF 208 atthe output of the first buffer stage 202 if the capacitor is integrated.This is because the low output impedance is often desired from amicrophone buffer, and the useful corner frequency of the LPF 208dictates use of a relatively large capacitance. Setting the cornerfrequency of the HPF 210 may also dictate use of a relatively largecapacitance, for example, on the order of several hundred Pico Farads.The input diodes 226 and 220 form a HPF with the transducer's motorcapacitance connected at VIN(not shown). The diodes are dc biased atground and have about 10 TOhms of resistance. This is the normal biasingscheme for microphone buffers.

The bias current of MNOUT 1 is set by a constant current source formedby second input transistor 222, which may be a depletion mode NMOStransistor and resistor 250. The output impedance of the first stagebuffer and resistor 240 form the R of the low pass filter 208. The C isformed by junction capacitor 242.

The second buffer stage 204 is driven by the HPF junction capacitor 246and is dc biased at ground through the HPF resistor 244. The HPFcapacitor 246 is made by the junction capacitance of the substrate ton-type isolation biased at VFILTER. The substrate is connected to theinput of the second buffer stage 204 and is therefore dc biased toground and driven with a buffered version of the input signal at theoutput of the LPF 208. This is a preferred implementation for guardingout the stray capacitance 252 from the output of the transducer 100connected at VIN and the substrate, because it allows for the substrateto isolation junction diode to be used as part of the HPF capacitor 246.This helps increase the die area utilization which helps minimize thestray feedback capacitor and thus electronic noise when using a floatingsubstrate design.

Advantageously, the filter capacitance may be provided by thecapacitance of the junction diodes formed using the isolation regionsavailable in many semiconductor manufacturing processes, including, forexample, most analog BiCMOS processes, may form one or more of thefilter capacitors. These junction diodes exist between the substrate andvarious isolation regions. This junction capacitor could be stacked withlinear capacitors or other circuitry in order to maximize thecapacitance per unit area. Significant die footprint reduction can beachieved using these junction capacitors in a floating substrate designto form the electronic filters, e.g., LFP 208 and HPF 210, in theimpedance buffer circuit 200 for the miniature electret microphone 100.

The use of junction capacitors in a floating substrate design helps tominimize die area impact of adding filtering functions to the integratedcircuit die 118. Typical values of the LPF 208 put the corner frequencyat 12 KHz which corresponds to the typical acoustic resonance frequencyfor a miniature electret transducer such as the microphone 100. Sincethe LPF resistor 240 (including the output resistance of buffer 202) arein the signal path of the buffered output, the resistor 240 (as well asthe output resistance of buffer 202) should be minimized in order toreduce the thermal noise contribution to the system noise. This makesfor a larger value capacitor 242 on the order of several hundred Picofarads in order to keep the electronic noise low. Since the HPF 210would benefit from a low value resistor 244 (around 5-10 MOhms) for goodnoise performance, a HPF corner frequency of 80 to 100 Hz requires thatthe HPF capacitor 246 needs to be several hundred pico farads as well.

In applications, such as in the miniature electret microphone 100, wheresensitivity improvements are made by reducing signal loss throughcoupling capacitances to the back of the die by floating the substrateand minimizing the die area, the use of junction capacitance for thefilters is advantageous. The nonlinear characteristics of these junctioncapacitors can be tolerated for most HI applications since the peak topeak signal amplitudes of the internal nodes in the buffer circuit 200are small. Since the buffer circuit 200 is an impedance buffer, thejunction capacitors may be biased with signals that are in phase withthe input signal, which would further reduce the risk of forward biasingany of the junctions integrated into the filters 208 or 210. Thevariation of bias voltage with signal may shift the corner frequenciesas well. This can also be tolerated in most HI applications, or can becompensated in other applications.

Junction capacitors utilized in a grounded substrate design for themicrophone 100 would not give the same benefits for minimizing signallosses. This is because of the loading effect on the input of the bufferto the substrate (ground). The benefit of increasing the die areautilization by using junction diodes in a grounded substrate would notaddress the gain degradation that would occur due to capacitive loadingin this situation.

Using linear capacitors in a floating substrate design would also not beas beneficial as using junction capacitors since the area of thecapacitor would grow the area of the die and increase the straycapacitance that exits from the input to the substrate. This would notbe optimal for reducing circuit noise because it does not minimize thefeedback capacitor.

A more efficient and preferred use of die area in a floating substratedesign would place the capacitors inside an isolation region, which isalso used as a junction capacitor to increase the utilization of thatarea.

It should be noted that both the LPF 208 and the HPF 210 could also becomprised of a linear capacitor wired in parallel with the junctioncapacitor and be contained in the same isolation region as the junctioncapacitor in order to increase the capacitance per unit area of thestructure.

FIG. 3 shows an illustration of the cross section of the LPF capacitor242. The series combination of the first stage's output impedance andthe resistor 240 drives the cathode of the LPF capacitor 242 which is ann-type isolation region 306 for the linear stacked capacitor 312 shownin FIG. 3. The linear capacitor 312 sits in a p-type well region 302whose junction capacitance with the n-type isolation layer 306 can beused as part of the LPF 208 or HPF 210 capacitor depending on if it isbiased to ground or to the input of the second stage. FIG. 3 shows abiCMOS process that has p-type epi layer 308 grown on a p-type substrate310. Similar structures can be made for a n-type epi process as shown inFIG. 4, where like reference numerals beginning with 4_identify likeelements. FIGS. 3 and 4 show a stacked poly capacitor 312 and 412,respectively. For the structure of FIG. 4, the poly to diffusioncapacitor is made with a p-type implant 414 instead of a n-type implant314 as shown if FIG. 3. In this situation, more capacitance per unitarea can be achieved by adding additional isolation junctions as shownin FIG. 5 where a p-type isolation layer 516 is added on top of then-type buried layer 506.

It will be appreciated that numerous variations to the above-mentionedapproaches are possible. Variations to the above approaches may, forexample, include performing the above steps in a different order.Further, more than one linkage assembly may be mounted within atransducer. In another example, the linkage assembly may be formed aspart of other components of the driving assembly. In yet anotherexample, other components of the driving assembly may be formed in asimilar manner.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextend as if each reference were individually and specifically indicatedto the incorporated by reference and were set forth in its entiretyherein.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventor for carrying out the invention. Itshould be understood that the illustrated embodiments are exemplaryonly, and should not be taken as limiting the scope of the invention.

1. A buffer circuit for a transducer comprising: an integrated circuitincluding a floating substrate and a p-n junction; and a filter circuitformed within the integrated circuit, the filter circuit including acapacitor formed by the p-n junction.
 2. The buffer circuit of claim 1,the filter circuit comprising a first filter and a second filter, thep-n junction comprising a first p-n junction and a second p-n junction,wherein the first p-n junction forms a first capacitor of the firstfilter and the second p-n junction forms a second capacitor of thesecond filter.
 3. The buffer circuit of claim 1 comprising a firstbuffer stage coupled to an output of the transducer and a second bufferstage providing a buffer circuit output, the filter circuit disposedbetween the first buffer stage and the second buffer stage.
 4. Thebuffer circuit of claim 1, wherein the p-n junction is formed within anisolation region of the integrated circuit.
 5. The buffer circuit ofclaim 1, comprising a linear capacitor coupled to the p-n junction. 6.The buffer circuit of claim 1, the capacitor being non-linear.
 7. Thebuffer circuit of claim 1, the p-n junction being biased in phase withan output signal of the transducer.
 8. A miniature electret microphonecomprising: a housing for the microphone including a sound inlet port; aback plate/diaphragm assembly disposed within the housing, the assemblyhaving an output responsive to sound pressure incident on the diaphragm;an integrated circuit including a floating ground substrate and a p-njunction disposed within the housing coupled to the output; and a filtercircuit formed within the integrated circuit, the filter circuitincluding a capacitor formed by the p-n junction.